If it were my project, I would try to remove the cover you are trying to join to, and draw a pattern on newspaper even if you have to tape several sheets together. I have not done any sail work, but I know the ends of canvas where it is hemmed can be quite thick to try and sew a zipper to (if I understand your need), so I would move the zipper just far enough in from the edge to be able to sew one side onto a single layer of the fabric. If I am wrong, try to take a closer photo and maybe I can help more...I am known as a "McGiver" with a sewing machine.